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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

Well, sticking 'em in /etc/hosts.deny will deny access (that's what DenyHosts does dynamically, plus other DenyHosts users are polled periodically and their bad-hats are added, plus DenyHosts purges /etc/deny.hosts every so often -- I have over 5,000 entries at present).

When these bastards get cranked up (you're getting hit by what looks an awful lot like compromised microjunk machines), other than pulling the plug there isn't all that much you can do but block entire countries (as in, do you really care if users in China or Korea can get to you?), use a tool like DenyHosts (which is pretty effective), block every non-essential port in your router, shut down SSHD, shut down FTP, shut down anything that responds to a ping (and don't ever allow a ping response in any event, that's one of the ways they find you).

Having a couple of thousand entries in /etc/hosts.deny doesn't hurt, can help and... what the heck, better safe than sorry, eh.